1. Field of the Invention
This invention relates to injection lasers, and in particular to antireflection coatings for pulsed high power injection lasers.
2. Description of the Prior Art
There is a limit to the peak power that can be generated by an injection laser. If this threshold is exceeded the output facet of the laser is destroyed. The disruption of this output facet has been attributed to local heating resulting from residual optical absorption in the material of the lasing filament. The surface damage is then explained on the basis that the heating is most severe at the output facet because this is where the optical intensity is greatest.
The ratio of this peak value of optical intensity to the light output of the laser is related to the reflectivity of the facet. Thus it has been predicted that treatment of this facet to reduce its coefficient of optical intensity reflectivity (R) would produce an increase in the destruction threshold. This prediction has been verified by practical experiment. The destruction threshold of an untreated laser has been reduced by a factor of about 2 by the use of a reflection reducing coating consisting of a layer approximately a quarter-wavelength thick of silicon monoxide. The electric vector amplitude reflection coefficient (r) of a GaAs - air interface is positive, and this use of silicon monoxide provides the surface with a reduced amplitude reflection coefficient (r). (The coefficient of reflectivity (R) is also reduced because R = r.sup. 2). Prior art articles discussing antireflection coatings for light emitting diodes and damage caused by high power laser pulses are found in the Journal of the Electrochemical Society, November 1963, pages 1124-1126 and Applied Physics Letters, Oct. 15, 1972, pages 364-366.
Optical feedback is however required for laser action, and hence there is a lower limit to the value of R which can usefully be employed. This provides an upper limit to the increase in destruction threshold which can be obtained with a positive electric vector amplitude reflection coefficient (r).